The present invention relates to carrying out a seismic survey in order to obtain data on the characteristics of lithological formations. This is done by bore drilling operations using measurements of the seismic signal as it propagates through the layers.
Such a determination is essentially based on the measurement of the detection times of reflected signals. These correspond to a signal generated by a seismic source. They are constituted by vibrations or elastic energy impulses, after the latter are reflected off of the geological layers at their various levels. In general terms, the determination system is constituted by a seismic source and a plurality of signal receivers for both direct and reflected signals. These are distributed around a well, and by using devices and procedures described herein for processing and interpreting the return signals detected by the receivers.
This detection technique is currently designated by means of its acronym "VSP" (Vertical Seismic Profiling). VSP makes possible a representation to be obtained of the underground adjacent to the drilled bore. This is done by starting from the signal reflections on the layers which constituted it. The position of the various underground layers is reconstructed on the basis of the delay times with which the signal returns to the receivers.
The techniques for the interpretation of the reflected signals have been considerably developed, and, with the aid of computer supported data processing, make it possible that information is obtained on the formations by bore drilling.
The conventional technique for VSP survey requires that the drilling operation be interrupted, and that the drill string and the drilling bit must be extracted from the well bore. One or more detection geophones are then introduced into the already drilled bore and one or more sound impulses are generated at either the surface level, or at a low depth, and in the nearby of the well (for example, by firing explosives charges). The signals are then detected. These signals reach the geophones which are installed inside the interior of the well. Such an operation does not cause uncertainties about the "primary" signal generated at the surface level. The return signals detected do not pose serious interpretation difficulties, but are affected by considerable drawbacks. Firstly, it is a very expensive operation because it requires that the drilling operations are discontinued for a long time interval, therefore, they can only be carried out an extremely small number of times during the course of the bore drilling. The operation of extraction and reinsertion of the drill string is very complex and not free from risks, and therefore requires that particular safety procedures and precautions are adopted.
More recently, it was proposed that such surveys should be carried out by using, as the seismic source, the same signal generated by the drilling bit during the drilling operation.
For example, such survey systems are disclosed in U.S. Pat. No. 4,965,774 to Atlantic Richfield, U.S. Pat. Nos. 4,862,423; 4,954,998 and 4,964,087 to Western Atlas Inc., U.S. Pat. No. 4,718,048 to S.N. Elf Aquitaine and U.S. Pat. No. 5,050,130 to Gas Research Institute.
Using the same drilling bit as the seismic source offers the advantage that surface measurements can be carried out during the bore drilling operations, without interfering with the drilling activities. A large amount of data can be collected with low cost and low risks, with frequent detection campaigns, or even continuously. Unfortunately, the signal generated by the drilling bit has the drawback of being affected by propagation disturbances and of being continuous over time. It is therefore very difficult to establish which is the reliable signal generated by the drilling bit to compare it to the return signals which are detected by the detectors installed on the ground in the area which surrounds the well being drilled.
In FIG. 1, the typical configuration of the characteristic parts of a drilling facility and of the seismic measurement collection system is shown, in which:
1 indicates the structure of the drilling derrick,
2 indicates the drill string, which, at is end, bears the drilling bit,
3 indicates the drilling bit,
4 indicates the rotary table which transmits the revolutionary movement to the drill string 2
5 indicates the electrical motor which actuates the rotary table 4,
6 indicates the mud tank,
7 indicates the delivery pump for the muds which flow along the interior of the drill string 2, down to the drilling bit 3, where they leave the drill string and rise back to the surface by flowing along the well,
8 indicates the well drilled by the drilling bit 3 during its downwards movement inside the underground, inside which the muds delivered with the pump 7 rise back to the surface level,
9 indicates a line of detection sensors 10 which receive both the direct seismic signals and the reflected seismic signals generated by the seismic source represented by the drilling bit, which flow through the ground and are collected and recorded by means of a recorder 11.
The detector lines are usually simply designated as the "seismic line" and are generally positioned at a certain distance from the drilling facility, according to criteria of optimization. This makes possible the seismic data to be acquired from a region with a certain surface area around the drilled bore. Such a seismic line is generally constituted by geophones, in the case when the bore is being drilled on land, or by hydrophones, in the case of off-shore drilling operations.
The return signal which is measured is bound to the signal generated by the drilling bit through a transfer function and generally is affected by noise. This seismic source offers great advantages if one succeeds in locating the bit signals, which are distributed over time and are disturbed by a strong environmental noise, for example, the noise generated by the other machinery pieces operating at the drilling facility and by pumps, and separating said signal from such a noise. In other words, the technical problem which the instant invention aims at solving is of obtaining a high enough signal/noise ratio in order to obtain meaningful information as to the nature and configuration of the lithological formations affected by the drilling work.
The amount of noise which is present in the primary signal causes a decay in detection quality, and the noise component must by removed as completely as possible from such a signal.
The present invention is based on adding the contribution of a large amount of data collected by the detection sensors during a discrete time period, to increase the signal/noise ratio. The length of the time period during which said data can be summed is limited by the fact that, during time, the drilling bit advances with simultaneously sinking. Such a technique finds applicative limits of practicality. For example, drilling softer or less compact layers generates weaker signals and measurements must be extended for longer time periods in order to be meaningful. Unfortunately, in such a case, drilling is faster and limitations must be faced as to the allowed time period length for the measurements, in order not to lose the space resolution, owing to the excessive sinking of the drilling bit during the measurements.
The noise prevents clear sensing of the bit signal and results in only being partially removable by means of the technique of adding the contribution of a large number of measurements repeated during discrete time periods. The result which can be accomplished is a better signal/noise ratio, but which is not yet high enough. The noise can be discriminated on the basis of its characteristics, for example, a certain regularity, or the fact of being characterized by random peaks. The bit signal contains unpredictable components due to the casualness of bit/rock coupling during the drilling operation. It has a self-correlation of pulsed type and can be distinguished from the environmental noise.